riscv timer: implement riscv_timer_et_stop()

Simply by masking timer interrupts.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35463

sys/riscv/riscv/timer.c

@@ -120,7 +120,8 @@ static int
 riscv_timer_et_stop(struct eventtimer *et)
 {
 
-	/* TODO */
+	/* Disable timer interrupts. */
+	csr_clear(sie, SIE_STIE);
 
 	return (0);
 }